Tumors express many immunogenic antigens, but in most cases the spontaneous response to these antigens by host T cells is poor. This represents a major problem for both immunotherapy and immunogenic chemotherapy. We now describe a population of highly immunogenic dendritic cells (DCs), arising in tumors during immunotherapy, which share attributes of ?conventional? CD103+ cDCs, but which also express lineage-markers of monocytic MDSCs. The scientific premise of the proposal is that this population of ?dual- phenotype? Ly6c+/CD103+ DCs is critical to anti-tumor immune responses during immunotherapy, and that it is possible to therapeutically amplify and increase these cells so as to markedly enhance the effect of immunotherapy. Aim 1 will test the hypothesis that the Ly6c+/CD103+ DCs arise via rapid differentiation of immature myeloid cells (monocytic MDSCs) in the tumor milieu, via a pathway that requires re-activation of classical DC-lineage transcription factors. The signal that initially drives this differentiation step is inflammation from dying tumor cells, but this is then rapidly amplified by inflammatory signals from activated T cells (positive feedback loop). The hypothesis predicts that Ly6c+/CD103+ DCs are critical for anti-tumor immune responses because they are the only cells in the tumor capable of re-activating anergic/exhausted T cells. Aim 2 will test the hypothesis that the key mechanism controlling differentiation of Ly6c+/CD103+ DCs is the transcription factor p53 expressed in MDSCs. The hypothesis predicts that p53 acts by inducing expression of the pro- inflammatory transcription factor IRF5; and that the upstream signal for p53 activation is the cell-intrinsic respiratory burst (reactive oxygen species) produced in response to inflammatory cytokines. Aim 3 will test the translationally-relevant hypothesis that pharmacologic activation of the p53 pathway, using clinically-applicable p53-agonist drugs, will drive the differentiation of increased numbers of Ly6c+/CD103+ DCs during immunotherapy, thus markedly enhancing and prolonging the anti-tumor immune response. To support translation of this approach to humans, this aim will also test the prediction that human myeloid DCs are controlled by an analogous p53-driven, IRF5-dependent maturation pathway. The translational importance of these findings is that they identify myeloid-lineage p53 as a previously unsuspected target for immunotherapy, which can be targeted by existing p53-agonist drugs already under clinical development for other indications.